Many catalytic processes are carried out in reactors that contain a series of separate catalytic beds. Reactors used in the chemical, petroleum refining and other industries for passing liquids or mixed-phase liquid/vapor mixtures over packed beds of particular solids are employed for a variety of different processes. Typical of such processes in the petroleum refining industry are catalytic dewaxing, hydrotreating, hydrodesulfurisation, hydrofinishing and hydrocracking. In these processes a liquid phase is typically mixed with a gas or vapor phase and the mixture passed over a particulate catalyst maintained in a packed bed in a downflow reactor.
In downflow reactors, it is necessary that gas and liquid are properly mixed and uniformly distributed across the horizontal cross section of the reactor prior to entering the catalyst beds. Uniform distribution helps ensure efficient utilization of catalyst, reduced catalyst top layer attrition, improved yields, improved product quality, and increased run lengths. Generally, in a multi-bed downflow catalytic reactor, a plurality of catalyst beds is arranged within the reactor and a distributor system for the proper mixing of gas and liquids is arranged in the region between two subsequent catalyst beds. This region is normally provided with a gas injection line underneath a catalyst bed, whereby additional gas is injected to compensate for the gas already consumed in the previous catalyst bed. The injected gas can also act as a quench gas. Generally, the injected gas is hydrogen or comprises hydrogen. The liquid falling downward from the above-lying catalyst bed is allowed to accumulate on a collector tray. The quench gas and liquid then pass into a mixing chamber where a swirling movement of the liquid is provided. This enables good mixing of the liquid and thereby even temperature conditions of the liquid. Gas-liquid mixing also takes place inside the mixing chamber. The fluid from the mixing chamber falls downward onto a deflector or impingement plate, whereby the flow is redirected onto a first distributor tray having a large number of downflow openings for the passage of liquid. For cross-sectional liquid flow distribution, the downflow openings can comprise one or more conduits, or chimneys. The chimney is a cylindrical structure with an open top and one or more openings in the upper portion of its height through which a gas phase can enter. The gas phase travels downward through the length of the chimney cylinder. The lower portion of the chimney can have one or more lateral openings for liquid flow through which a liquid phase can enter the cylindrical structure of the chimney and contact the gas phase. As liquids accumulate on the distributor tray, they rise to a level that covers the lateral opening or openings in the chimney so that the passage of gas is precluded and so that the liquid can enter through the lateral opening or openings into the cylindrical structure. Gases and liquids egress via an opening in the bottom of the chimney, through the distributor tray, and onto an underlying catalyst bed. Only limited mixing between the two phases happens in the cylindrical structure because of the low turbulence around liquid streams.
A good flow distribution device should meet the following four basic requirements: provide even distribution of feed to a catalyst bed over a range of gas and liquid rates; be tolerant to certain out-of-levelness of the distribution tray; provide good gas-liquid mixing and heat exchange, and require minimum catalyst bed height to fully wet the underlying catalyst bed. Because the driving force for liquid flow into the chimney is the static liquid height on the tray, standard chimneys can be deficient in meeting these criteria due to poor tolerance for deviations from levelness of the distributor tray. They also suffer from suboptimal spray discharge of fluids onto the underlying catalyst bed.
One of the key considerations in flow distributor design is the discharge pattern of liquid and gas from the device. A standard chimney distributor provides only some point contacts of liquid with the catalyst bed. As a result, it takes certain bed height to adequately wet the catalyst surface and for the desired catalytic reactions to occur. A more uniform and consistent spray pattern and more uniform catalyst wetting in a short length of catalyst bed are desired. It is an object of this invention to achieve an even distribution of fluid over the top of the catalyst bed as a sustained spray. It is another object of the invention to improve the tolerance for flow distributor design for distributor tray out of levelness.